From treating patients by dialysis devices, it is known to heat dialysis fluid before the fluid flows into the dialyzer or blood treatment filter in which substances, e.g., blood and dialysis solutions, are exchanged through a membrane which usually is of a semi-permeable type. Some of these heating devices require achieving a secure contact or require keeping a maximum distance between the heating device, which may for example be shaped as heating coils or heating spindles, and a container containing the dialysis fluid to be heated, in which the container is surrounded by said heating device and the container may by way of example be embodied as a bag. With a heating device of this type, heating the dialysis fluid as intended depends on how well the container contacts the heating device, which may, for example, be embodied as a bag heater (or vice versa).